The invention relates to a process for reducing iron-ore-containing particulate, in particular fine-particulate, material in at least a two-stage process, wherein reducing gas is conducted through at least two reaction zones consecutively arranged in series and formed by a moving particulate material. The particulate material passes through the reaction zones in reverse order to the reducing gas. The particulate material is heated in the reaction zone arranged first for the particulate material and is reduced in the further reaction zone or in the further reaction zones, respectively.
A process of that kind is known from U.S. Pat. No. 5,082,251, WO-A-92/02458 and EP-A-0 571 358. According to U.S. Pat. No. 5,082,251, iron-rich fine ore is reduced in a system of fluidized-bed reactors arranged in series by aid of a reducing gas under elevated pressure. The thus produced iron powder is then subjected to hot or cold briquetting.
The reducing gas is produced by catalytic reformation of desulfurized and preheated natural gas with superheated water vapour in a conventional reformer furnace. After this, the reformed gas is cooled in a heat exchanger and, subsequently, the H2 portion in the reducing gas is increased by CO conversion with the aid of an iron oxide catalyst. Subsequently, the CO2 forming as well as the CO2 coming from the reformer are eliminated in a CO2 scrubber.
This gas is mixed with the reducing gas (top gas) consumed only partially, is heated, and the fine ore is reduced in three steps (three fluidized-bed reactors) in counterflow.
The ore flow starts with drying and subsequent screening. Then, the ore gets into a preheating reactor in which natural gas is burnt. In three consecutive reactors, the fine ore is reduced under elevated pressure.
From EP-A-0 571 358 it is known to carry out the reduction of fine ore not exclusively via the strongly endothermic reaction with H2 according toFe2O3+3H2=2 Fe+3H2O−ΔH, but additionally via the reaction with CO according toFe2O3+3CO=2 Fe+3CO2+ΔH, which is an exothermic reaction. Thereby, it is feasible to considerably lower the operational costs, in particular the energy costs, involved.
According to the prior art, direct reduction, because of the kinetics of the known processes, involves magnetite formation during direct reduction in a layer constantly growing from outside towards inside and forming on each particle or grain of the iron-oxide -containing material. It has been shown in practice that the formation of magnetite has an inhibiting effect on direct reduction with a reducing gas. Thus, it is feasible only at elevated expenditures to obtain a more or less complete reduction of the iron-oxide-containing material charged.
The reaction kinetics of magnetite formation is influenced by the composition of the gas and of the solid. The molecules of the reducing gas must get from the outer gas flow through the adhering gas border layer and through the macropores and micropores to the site of reaction. There, the dissociation of oxygen takes place. The oxidized gas gets back on the same way. The ore grain is, thus, reduced from outside towards inside. Thereby, its porosity increases, since the dissociated oxygen leaves hollow spaces and the original volume of the ore grain hardly shrinks. The reaction front migrates from outside towards inside into the ore grain. With dense layers, the concentration of the reducing gas decreases from outside towards inside. The gas at first diffuses from outside through the already reduced shell as far as to the reaction front, where it is reacted and then diffuses back as a reaction product. With porous surfaces, the phase border reaction occurs on the walls of the pores within the reaction front, while the gas at the same time also may diffuse inside. With dense magnetite layers on the surface of the ore grain, the reaction kinetics is inhibited because the reducing gas is impeded from diffusing by exactly that layer and the mass transfer of the reducing gas thus cannot occur in the same manner as with porous ore grains.
The formation of a magnetite layer occurs very rapidly, i.e., the more rapidly the closer the temperature of the iron-oxide-containing material is to the limit temperature of about 580° C. According to the Baur-Glaessner diagram, such a formation of a dense magnetite layer on the surface of an iron ore grain primarily occurs up to a temperature of the iron ore of 580° C. upon contact with the reducing gas. At a temperature of the iron ore of below 400° C., the formation of magnetite is slowed down despite the contact with the reducing gas, and, as a result, dense magnetite layers are formed less rapidly.
In order to convert the previously mentioned magnetite formation or mixtures of magnetite and metallic iron, respectively, in further reduction steps to a largely metallic condition, longer retention times in the subsequent stage(s) of treatment or higher solid and/or gas temperatures are necessary.
Those phenomena lead to a number of disadvantages:                the formation of more abrasion from the solid due to longer retention times in the subsequent reduction stages,        an intensified metallization of this abrasion and hence increased tendency toward agglomeration of that material,        a substantial deterioration of the fluidization properties of the fluidized beds formed from that material,        the formation of adhesions in cyclones, downpipes (diplegs) and transport ducts (standpipes),        hence problems with the transfer of products        a low metallization of the final product.        
Furthermore, those occurrences involve an increased need for reduction, a higher dust discharge (and hence an increased oxide consumption) and a larger amount of waste. It is particularly necessary to provide a reducing gas having a high reduction potential also in the fluidized-bed zones arranged to be first.
From WO-A-99/09220, it is known to adjust the temperature of the iron-oxide-containing material in the first fluidized-bed zone to either below 400° C. or to above 580° C. or to adjust the temperature to a range of from 400 to 580° C. in order to utilize the reducing gas both in regard of its reduction potential and in regard of its sensible heat, wherein, at a temperature adjustment to below 400° C., the temperature range of between 400° C. and 580° C. in the fluidized-bed zone arranged to follow the first fluidized-bed zone in the flow direction of the iron-oxide-containing material is passed through within a period of 10 minutes, preferably within 5 minutes, and wherein, at a temperature adjustment to above 580° C., the temperature range of between 400° C. and 580° C. is passed through within a period of maximally 10 minutes, preferably 5 minutes, and wherein, furthermore, at a temperature adjustment in the range of from 400° C. to 580° C., the iron-oxide-containing material remains within that temperature range for a maximum of 10 minutes, preferably 5 minutes, and is passed on into the fluidized-bed zone following next immediately after having reached the desired temperature.
By those measures, it is feasible to reduce the formation of magnetite layers down to a tolerable degree.
According to the process described in WO-A-99/09220, the transition of the temperature of the iron-oxide-containing material during heating from 400 to 580° C. is accomplished within as short a period of time as possible and maintenance within that critical temperature range is avoided. When rapidly passing that temperature range, the formation of a magnetite layer is extremely modest despite a reducing gas exhibiting a high or optimum reduction potential, respectively.